1. Field of the Invention
The present invention is directed to a method and apparatus for robbing bits in an ATM cell stream, and more particularly, to inserting information into the robbed bits to form a new robbed bit stream.
2. Description of the Related Art
A standard format for digital video transmission has not yet been adopted. A standard for ATM access has also not yet been adopted. In the United States, a 6 MHz channel plan for National Television Standards Committee (NTSC) video broadcast has been the accepted standard. The desirability of incorporating other information streams, e.g., teletext, into the digital video stream has been recognized, but has not been realized. However, whatever the standards to be adopted in the future, maximum use of an ATM cell will be required.
ATM access makes use of ATM cells to transmit information from the headend of a system to, for example, individual homes in a serving area. The ATM standard provides a fifty-three (53) byte cell including a payload of 48 bytes and a five byte header. The five byte header includes one byte for header error control (HEC), a status half-byte, including payload type bits (PT) and cell loss priority bits (CLP). The remaining three and one-half (3xc2xd) bytes are apportioned in two different ways depending upon where the signals are in the overall network. These two interfaces are known as a user to network interface (UNI) and a network to network interface (NNI). According to the ATM standard, all 3xc2xd bytes (28 bits) are used for addresses at the NNI while according to the ATM standard 3 bytes (24 bits) are used for addresses at the UNI and the remaining half-byte is reserved for generic flow control (GFC). However, in an access environment, only 10-12 bits are needed for addresses. The remaining 12 to 18 bits need not be used for ATM addressing.
The present invention provides a method and system for robbing bits from a header in an ATM cell. An ATM switch at an office assigns and places information into the header portion of an ATM cell stream. Not all of the bits in the header portion of the ATM cell stream are used. A circuit xe2x80x9crobsxe2x80x9d the unused bits from the header portion and then inserts information into the unused bits in the header portion. A terminal unit at a subscriber end is provided for extracting the information from the unused bits and inputting the information to various information appliances.
The above-mentioned features and advantages of the present invention are achieved by employing a method providing an ATM cell stream, identifying unused portions of the ATM cell stream, robbing bits from the unused portions of the ATM cell stream to form a robbed bit stream, inserting information into the robbed bits, and extracting the information from the robbed bit stream at a subscriber end.
In addition, a system is provided including an ATM switch for outputting an ATM cell stream, a unit for robbing bits from the cell stream, a unit for inserting information into the robbed bits of the cell stream and a unit for extracting the inserted information.
The above features provide the advantages that additional information can be carried, such as, for example, a program guide or an additional broadcast video channel.